The present invention relates to a rotor and in particular to a rotor with permanent magnets for an electric motor.
Electric motors are used in many applications. Smooth running of the motor is particularly important for applications necessitating high rotational speeds, such as in electric motors for driving a radial compressor. This means that the motor is to run as smoothly as possible, even at high rotational speeds, so that the same does not transmit any vibrations to the overall system in which the motor is incorporated.
Normally, in rotors used for such motors, permanent magnets are mounted on a rotor shaft. Here, for example, a metal rotor shaft is used. Then, two or more permanent magnets are applied to this rotor shaft. These permanent magnets can, for example, be adhered onto the metal rotor shaft, which is produced, for example, of tool steel. Particularly at high rotational speeds, it is important that the permanent magnets are mounted sufficiently tight on the rotor shaft, so that no slip occurs between permanent magnets and rotor shaft and that, on the other hand, the permanent magnets do not become detached from the rotor. Particularly at high rotational speeds, centrifugal forces acting on the permanent magnets are extreme. Shear forces between permanent magnets and rotor shaft are also high, in particular when the motor is under load, i.e. when forces occur that want to “twist” the permanent magnets with respect to the rotor shaft.
Above that, such permanent magnet synchronous motors are driven in that a rotating magnetic field is generated by a stator having at least three stator coils, which drives the rotor with the permanent magnets. This is caused due to the fact that the “instantaneous” magnetic field situation within a stator “runs ahead” of the orientation of the magnetic field fields of the rotor in the rotating direction of the motor, such that the rotor is “pulled behind” by the continuously running ahead magnetic field generated by controlling the stator coils.
Such a permanent magnet synchronous motor can also be operated as electric generator. Here, the rotor is driven by mechanical force, and the movement of the rotor with its permanent magnets effects an induction voltage in the at least three stator coils.
Such an exemplary rotor is shown in FIG. 6. FIG. 6 shows a rotor shaft 100 to which four schematically illustrated permanent magnets 101, 102, 103, 104 are applied. The individual permanent magnets are applied in 90° sectors and magnetized such that alternating magnetic north poles N and magnetic south poles S are arranged outside and inside, as shown schematically in FIG. 6. If a rotor shown in cross-section in FIG. 6 is rotated within a stator comprising at least three coils, an almost sinusoidal electric induction voltage can be sensed at each coil terminal.
The rotor shown in FIG. 6 is not ideal for different reasons.
One reason is the reduced mechanical stability. Due to the shear forces between the surface of the rotor shaft 100 and the adjacent surface of the permanent magnets, a slip can occur between the ring of permanent magnets on the one hand and the rotor shaft on the other hand, or a very high load is applied, for example, to the used adhesive connection. This can have the effect that the permanent magnets become partly or completely detached from the rotor shaft, which can have the effect, in particular at high rotational speeds, that the permanent magnets become partly detached and hit the adjacent stator elements and result in a destruction of the motor.
A further reason is that the induced voltage in the three stator coils is sinusoidal in case of a generator operation, and that also the voltage to be applied to the three stator coils for operating the motor in motor operation will be sinusoidal. Switching a sinusoidal voltage, however, is generally and in particular in a digital environment disadvantageous.